Light supplies energy to plants and in the process of photosynthesis, converts it to useful forms of chemical energy which in turn is used to build up complex chemical constituents, notably carbohydrates which both plants and animals require. Light is one of the major plant growth factors in the ambient environment, and in many instances may be the limiting factor in plant growth. Much attention has been given to the control of temperature, humidity, water, and nutritional requirements of plants, but until recently there was little that could be done on the control of light. This has not been due to any lack of appreciation of its importance, or the lack of interest in the problem, but was due largely to inefficient light sources and growth responses to light that were not too well understood.
In recent years electric light sources have become an important tool to the plant biologist and commercial grower, to provide the daily irradiation necessary for the plant's normal development.
In the northern latitudes of North America and Europe light quantity is a limiting factor for the growth of plants in greenhouses during the winter months. Different types of lamps have been used in attempts to increase light intensity during this period, e.g., fluorescent, fluorescent plus incandescent, neon, sodium, mercury-tungsten, metal halide, and mercury fluorescent. A mercury fluorescent lamp with reflective coating specifically designed for horticultural purposes has also been used successfully.
A metal halide lamp with its high lumen output and a spectrum which matches the plant growth action spectra would improve plant growth even more.